1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus, particularly to a sheet stacking apparatus that can align a sheet stacked on a sheet stacking portion and an image forming apparatus provided therewith.
2. Description of the Related Art
Conventionally, there is well known a sheet stacking apparatus that aligns the sheet, which is discharged to the sheet stacking portion that stacks the sheet thereon, in a sheet width direction orthogonal to a discharge direction to improve a property of taking the sheet on which an image is formed (see U.S. Patent Application Publication No. 2002/0079642 A1).
The sheet stacking apparatus disclosed in U.S. Patent Application Publication No. 2002/0079642 A1 includes a pair of aligning members that can rotate about an upper part of the sheet stacking portion to align the sheet in a lower end part, and each of the pair of aligning members is abutted on an end face in the sheet width direction to perform the alignment in sheet width direction. For example, in the sheet stacking apparatus, one of the pair of aligning members is moved in the sheet width direction, and the sheet is pressed against the other aligning member that is of a reference, thereby performing the alignment in the sheet width direction. The sheet stacking apparatus can also align and sort each of an unbound sheet bundle to which a stapling process is not performed in a position shifted (deviated) in the sheet width direction as needed basis.
The sheet is discharged to the sheet stacking portion not only one by one but also in units of sheet bundles. Therefore, generally a discharge portion that discharges the sheet can swing vertically such that an opening amount of the discharge portion can be changed according to a thickness of the discharged sheet bundle, and the pair of aligning members is disposed above the discharge portion so as not to interfere with the sheet bundle discharged from the discharge portion.
In the sortation stack in which subsequent sheet bundle is aligned in the position shifted in the sheet width direction with respect to the previously-stacked sheet bundle, sometimes the sheet aligned in the sheet width direction passes below the pair of aligning members when the sheet of the subsequent sheet bundle is aligned one by one by the pair of aligning members. In order to prevent the trouble, it is necessary that, in performing an aligning process, the sheet-pressing-side aligning member be located below the reference-side aligning member placed on the previously-stacked sheet bundle. However, as described above, the pair of aligning members is disposed above the discharge portion, and rotatably supported with the upper part of the discharge portion as a rotating center. Therefore, when the pair of aligning members is rotated such that one of the aligning members is moved downward, a position of an alignment region of the pair of aligning members in the discharge direction changes by a rotating radius of the aligning member with respect to the discharged sheet. When the sheet is nipped by the pair of aligning members, which are deviated from each other in the discharge direction, a torque is provided to the sheet in pressing the sheet, and possibly the sheet is inclined.
An object of the invention is to provide a sheet stacking apparatus including a pair of aligning members that can suitably align the sheet in the sheet width direction orthogonal to the discharge direction and an image forming apparatus.